1. Field of the Invention
The invention relates to a separately excited electric machine having a rotor and a stator as well as a device for the detection of the rotational position of the rotor relative to the stator, wherein the position determination device is designed for determining the rotational position of the rotor by evaluating a signal which can be tapped at the pole winding circuit and is imprinted by pole winding inductivities having rotational position dependency.
2. Description of the Related Art
In known separately excited electric machines of this kind the device for the detection of the rotor-stator-position comprises sensors, for example, Hall generators, which detect the field orientation or/and field strength of the exciting magnetic field, wherein the rotor position relative to the stator is determined as a function of these measured values. The installation of such sensors in electric machines increases their manufacturing expenditure. Furthermore, there is often the disadvantage that a calibration of the sensors must be carried out. Finally, the arrangement of such sensors can also cause a disturbance of the field extension within the motor which can impede the proper running properties of the electric machine.
An electric machine of the aforementioned kind is known from U.S. Pat. No. 5,254,914. For determining the rotational position of the rotor while the rotor is standing still, short current pulses are sent through selected phase strands of the pole winding circuit and the subsiding inductive voltages occurring in response to the pulses are measured. The rotational position of the rotor is determined based on the basis of the sign of the difference between the subsiding time periods and the inductive voltages.
The present invention has the object to provide a novel separately excited electric machine of the aforementioned kind which allows a reliable determination of the rotational position of the rotor with a simplified circuit configuration.
The separately excited electric machine according to the invention that solves this object is characterized in that the pole winding circuit has phase strands that are star-connected and that the position determination device is designed for evaluating the potential at the star point.
Rotational position dependencies of the pole winding inductivities result at the star point, which according to the invention is the only measuring point, in potential fluctuations which are detectable measuring-technologically with minimal circuit expenditure and can be easily evaluated for determining the rotational position of the rotor with the aid of pre-stored correlating relationships between the potential and the rotational position.
The dependency of the rotational position of the pole winding inductivities can be based on the changes of the magnetization of pole winding cores caused by the excitation field. Depending on the relative arrangement of the pole winding cores and the excitation field, different magnetization of the pole winding cores may occur based on the dependency of the induction flow, that is controlling in regard to the inductivity, on the magnetic field strength of the excitation field, wherein these different magnetizations have an effect on the inductivity of the pole windings.
As an alternative, or in addition thereto, an arrangement of magnetic materials can be provided which changes the inductivity of the pole windings as a function of the rotational position. Depending on the rotational position, the respective contribution of the magnetic material arrangement in regard to the inductivity of the pole windings changes.
In one embodiment of the machine, the material arrangement is formed by a field magnet support ring that serves in particular for forming the rotor, wherein along the circumference of the field magnet support ring the material thickness or/and permeability of the ring material can change. The material thickness could, for example, change in that the field magnet support ring is circular at its inner side supporting the field magnets, while its outer side has a spirally shaped mantle with steadily increasing thickness. In this case, a univocal dependence of the pole winding inductivity from the rotational position can be achieved in the area of a complete rotation of 360xc2x0. A two-fold defined dependency about 360xc2x0 could be achieved by forming the outer surface of the ring with a shape corresponding to an unsymmetric oval.
In a further embodiment of the machine, the excitation field and/or the material arrangement are designed for generating a dependency course of the pole winding inductivity from the rotational position, which returns periodically with the rotor rotation, are provided, wherein the position determination device for determining the rotary position for overlapping periods can comprise a counting device that will detect the recurrence of dependency periods beginning at a starting point. The periodically returning dependency course can result from several sequential magnetic periods that form the excitation field. On the other hand, the material thickness of the support ring could change by securing cutouts for field magnets distributed in the circumferential direction. A position determination based on the aforementioned dependency then takes place respectively within a certain rotational angle period of the rotor, while the counting device determines the respective rotational angle period of the rotor and thus the position of the rotor relative to the stator over the entire rotational angle range of 360xc2x0.
In the case of formation of a magnetic period by two field magnets positioned in a respective securing cutout, a univocal dependency would result respectively within a half magnetic period, wherein in a multi-pole machine this dependency would repeat according to the number of magnetic periods distributed over the circumference. When the securing cutouts are extended to one or more periods, univocal dependencies result respectively within the expanded angular areas.
Preferably, the position determination device is provided for producing a measuring voltage that is applied, in particular selectively via respective phase strands, to the pole windings and has a frequency that is preferably above that of the operational voltage frequency, wherein an evaluation device is provided with a filtering device for filtering a signal, representative of the momentary strand inductivities, from the star point potential.
In an especially preferred embodiment of the invention, a pulse switching is provided for the operation of the machine, especially with pulse width modulation, wherein for the purpose of signal detection the position determination device evaluates the course of the potential as a result of the voltage pulses. During pulse operation of the machine a fluctuating potential course results at the star point with increases varying according to the momentary pole winding inductivities.